The present invention relates to electric machines such as, for example, motors, generators, alternators, starter-generators, and the like; and, more particularly, the present invention relates to electric machines having redundant windings with current limiting means.
One example of an electric machine is a permanent magnet electric motor or generator. This machine includes a rotor formed, at least in part, from a permanent magnet material such as samarium-cobalt. In a motor, current through the windings induces a rotating magnetic field, which in turn applies a torque to the magnetic portion of the rotor causing it to act as motor. Similarly, in a generator, torque applied to the rotor results in a rotating magnetic field that induces a current in the windings.
Such electric machines provide significant benefits over wound field synchronous machines, squirrel cage motors and other types of electric machines that require brushes. Significantly, permanent magnet machines do not require brushes, are relatively reliable and light, use electronics to generate any required rotating magnetic field, and can act as both a motor and a generator.
In view of these benefits, such machines appear well suited for aircraft applications. Particularly, such machines would appear to lend themselves for use as starters and generators within a turbine engine. Conveniently, such machines can be connected directly to the engine shaft. When required, generated electricity can be rectified and filtered using conventional lightweight electronics. When DC power is required, as in traditional aircraft applications, the speed of rotation and frequency of generator output do not need to be controlled. Such machines can function as both starters and generators.
Due to the heat generated by high short circuit currents in permanent magnet machines, the windings in permanent magnet machines must be sufficiently protected from the unlikely event of internal faults. Protective external circuits and equipment are often provided with fusible materials or electronic controls external to the windings of the electric machine; however, internal short circuit conditions may occur within the windings of a motor/generator that would not be detected or controlled by external fusing or controls. While various approaches have been developed to protect the windings from high short circuit currents, these approaches do not provide redundancy and safety features.
For example, one approach to providing protection within the windings is described in U.S. Pat. No. 6,313,560, which is incorporated by reference herein in its entirety. The '560 patent describes an electric machine including a magnetic component, forming part of its rotor or stator, that loses its magnetic characteristics above a certain chosen temperature. This magnetic material forms part of a magnetic circuit that guides flux about the stator. As a result, any magnetic flux emanating with the rotor stops circulating about the stator above this temperature, and the machine stops acting as a generator or a motor. The material forming the component is selected so that the chosen temperature is lower than the temperature at which the machine would be thermally damaged. This, in turn, limits the operating temperature of the windings, and thus prevents overheating of the machine during operation typically caused by a fault. Problematically, once the temperature is exceeded, the machine stops working until the machine cools down. Thus, there is a period where the machine provides no power.
Another approach is described in US Patent Application Publication No. 2004/0189108, which is incorporated by reference herein in its entirety. This publication describes a permanent magnet electric machine wherein the material of the stator is selected such that magnetic flux circulation through the stator is impeded when the stator material acquires a temperature above its Curie temperature. Like the aforementioned '560 patent, once the temperature is exceeded, the machine stops working until the machine cools down.
Yet another approach is described in US Patent Application Publication No. 2004/0183392 and US Patent Application Publication No. 2004/0184204, both of which are incorporated by reference herein in their entirety. These publications describe various means for limiting maximum current conducted through windings of an electric machine having a rotor and a stator. By encouraging an appropriate leakage flux around a winding, a leak impedance can be achieved which may be used to limit the maximum current in the winding as a matter of machine design. The means described in these publications, like slot geometry including extremely small slot gap width, large radial tooth top height, and use of top sticks made of material having high magnetic permeability, have been extensively used for over 20 years in various engine-dedicated types of permanent magnet alternators used on turbine engines to effectively increase the inductance. Problematically, such means do not provide for winding redundancy. That is, if one phase of the windings is shorted, the remaining phases do not continue to provide the same electrical output as before.
In summary, there remains a need for a method and apparatus for limiting short circuit current in electric machines that will allow the machine to continue to work, at least at reduced capacity, during and after the short circuit condition. Furthermore, there remains a need for a method and apparatus for limiting short circuit current in electric machines that will provide magnetic and electrical isolation of the windings, thereby providing winding redundancy and reliability under fault conditions. Prior art methods do not fulfill these needs.